In optical space communication, particularly, in ultra-long-distance optical space transmission such as optical space communication between satellites, it is general to use transmission light in a single polarization state. This is in order to perform bi-directional communications, in which a single optical link path is established in the free space by tracking transmission light released from the own station and transmission light released from a partner station with use of an optical antenna of the own station and an optical antenna of the partner station.
In the ultra-long-distance optical space transmission, it is necessary to separate, in a wide dynamic range, transmission light of a strong optical power transmitted by the own station from reception light of a weak optical power transmitted by the partner station. As some of the methods, there are proposed a method in which transmission light and reception light are separated from each other by using wavelengths different from each other, a method in which transmission light and reception light are separated from each other depending on a polarization state of transmission light of the own station and a polarization state of transmission light of the partner station, and a method in which the aforementioned methods are combined.
For instance, PTL 1 discloses a technique, in which transmission light of a strong optical power transmitted by the own station and reception light of a weak optical power transmitted by the partner station are separated from each other depending on a polarization state. FIG. 6 is a block configuration diagram of an optical space communication device of PTL 1. An optical space communication device 90 illustrated in FIG. 6 is configured such that linearly polarized transmission light generated in an optical transmitter 91 is separated by a beam splitter 93 depending on a polarization state, P-polarized transmission light is output to an optical monitor 94, and S-polarized transmission light is output to a collimator lens 95.
The optical monitor 94 adjusts the optical axis of a transmitted beam, based on the input P-polarized transmission light. On the other hand, the S-polarized transmission light output to the collimator lens 95 is collimated into parallel light beams by the collimator lens 95. Thereafter, the parallel light beams are converted into circularly polarized light by a quarter wave plate 96. After the beam diameter of the circularly polarized light is expanded by a sub mirror 97 and a main mirror 98, the expanded beam is released into the free space.
Further, when circularly polarized reception light is irradiated from the partner station, the optical space communication device 90 illustrated in FIG. 6 reduces the beam diameter of the irradiated reception light by the main mirror 98 and the sub mirror 97, and converts the circularly polarized light into linearly polarized light by the quarter wave plate 96. Further, the converted linearly polarized reception light is collimated into a parallel light flux by the collimator lens 95. Thereafter, the parallel light flux is input to the beam splitter 93 and is transmitted through an optical receiver 92 based on the polarization state for demodulation.